1. Field of the Invention
This invention relates to improved processes for making the products commonly known in commerce as "alkyl naphthalene sulfonates", even though they also may contain unreacted naphthalene, alkyl naphthalenes, and molecules with more than one alkyl group and/or sulfonate group per naphthalene molecule. The products are commercially important surfactants, particularly for agricultural use, and are normally made by reacting naphthalene with alcohols, sulfuric acid, and oleum.
2. Statement of Related Art
Butyl naphthalene sulfonate is now commercially made by reacting naphthalene, normal butanol, concentrated sulfuric acid, and oleum in a one-step batch type process. The process normally experiences a vigorous exotherm that is difficult to control and produces large amounts of sulfur containing by-products. On the other hand, isopropyl naphthalene sulfonate is now commercially made by a two-step process, with sulfonation followed by alkylation. While easier to control than the one step process, this two-step process is notably slower and also consumes more acid for by-products than is desirable. Similar situations prevail for other alkyl naphthalene sulfonate products.
After any of these syntheses, the initially formed alkyl naphthalene sulfonic acid is usually converted to the desired surfactant by dissolution in aqueous alkali to convert the acid to a salt. The surfactant may be used directly in the aqueous solution thus formed, or the solution may be dried to produce solid surfactant.
A material known in the art as "free oil" is a common but undesirable constituent of commercial alkyl naphthalene sulfonates. This material is largely unreacted naphthalene and/or unsulfonated alkyl naphthalene(s), and it is common commercial practice to impose an upper limit on the amount of free oil that is acceptable in the product. Another common and undesirable constituent of commercial alkyl naphthalene sulfonates is sulfate salts, formed during neutralization from residual sulfuric acid in the initial reaction product. Limiting the amounts of both these constituents is therefore a desirable goal of any process for making alkyl naphthalene sulfonate surfactants.
It is an object of this invention to provide a process for making alkyl naphthalene sulfonates that avoids or reduces at least some of the difficulties and/or byproducts occurring with present commercial processes.